In outboard engine units, various metal portions are exposed to seawater, and thus, metal members more prone to corrosion than the metal portions are sometimes attached to the metal portions with a view to preventing corrosion of the metal portions. The “metal members more prone to corrosion” are often called “sacrificial electrodes” or “sacrificial anodes” because they are more easily ionizable and have a lower positive potential than the metal portions and dissolve due to corrosion in place of the metal portions. Examples of the conventionally-known mounting structures for such a sacrificial electrode include one where a mounting port with a mounting hole opening to a water jacket of a cylinder head is closed with a cover, and where a sacrificial electrode is attached to the cover (see, for example, Japanese Patent Application Laid-Open Publication No. H10-236390).
FIGS. 4 and 5 show the sacrificial electrode mounting structure disclosed in the H10-236390 publication. As shown, the water jacket 102 is provided in the cylinder head 101, and circular holes 103 and 104 are formed in the cylinder head 101 in such a manner that the holes 103 and 104 open to the water jacket 102. An anode 107 functioning as the sacrificial electrode is inserted in the water jacket 102 and fixedly fastened, via a bolt 108, to the cover 106 that closes the circular holes 103 and 104 and is fixedly mounted to the cylinder head 101 by means of a bolt 111. However, because the cover 106 has a large mounting area, it sometimes can not be properly mounted in a small space of the cylinder head 101.
In order to prevent the aforementioned special problem, a sacrificial electrode mounting structure shown in FIG. 6 has been proposed, in which a lid member 118 is screwed to a mounting port 117 that opens to a water jacket 116 of an engine 115 and in which an anode metal member 122 functioning as a sacrificial electrode is fixedly fastened, via a screw 121, to a distal end portion of the lid member 118. However, because the lid member 118 is screwed to the mounting port 117 with a sealing tape wound on an external thread of the lid member 118 in order to seal an engagement portion between an internally threaded portion of the mounting port 117 and the externally threaded portion of the lid member 118, part of the sealing tape may sometimes undesirably protrude outside the engagement portion to get into between a flange 118a of the lid member 118 and a washer 123 or between the washer 123 and an end surface 117a of the mounting port 117. As a consequence, an electrical connection between the mounting port 117 and the lid member 118 (more specifically, the anode metal member 122 fixed to the lid member 118) tends to become poor, which may result in contact failure of the sacrificial electrode (i.e., anode metal member 122). Thus, it would become difficult for the anode metal member 122 to corrode because electric corrosion of the anode metal member 122 is hindered.